The present invention relates in general to injection molding methods, designs, and structures, and, more specifically, to over-molding methods for producing injection molded hollow articles. Exemplary uses of molded articles include inflatable bolsters to mitigate injury in a vehicular collision. Described are bolster designs and methods for molding and tooling in bolster production.
An active bolster is a vehicle occupant protection device with a gas-inflatable bladder to absorb impacts and reduce injury to occupants during a crash. Active bolsters deploy in a vehicle crash to cushion the impact force of an occupant against an interior panel of the vehicle. As opposed to air bag cushions, that emerge from behind various openings upon inflation, active bolsters use the interior trim surface itself to expand upon sensing a crash event to absorb an occupant impact and dissipate energy by venting an inflation gas.
The bolster has an expandable hollow chamber typically formed by injection molding a vehicle interior-facing front trim wall section and a rear bladder section. The front and rear sections may be attached around the periphery to join the two sections forming the chamber. This manufacturing process provides the advantages of injection molding which include: providing materials of higher strength and consistency than blow molding, producing parts with good fit and finish, and forming each section separately such that a different material and a different thickness of material may be used in making each section.
However, this manufacturing process also has drawbacks. The separate formation and subsequent alignment and joining of the parts increases the time and labor needed to manufacture the bolster. The weld seam between the two sections creates a weak point which increases the possibility of weld separation during inflation or increases the rejection rate of bolster units during production. The process of hot-plate welding generally involves forming the weld on a flat surface such that the direction of the inflation force is perpendicular to the weld which can lead to peeling apart and weld separation when the bolster inflates.
Various stresses during inflation can contribute to the possibility of weld separation at the attachment joint between separately molded structures. Due to the configuration of attachment for making the pleated bladder wall inflatable, a significant peel stress is experienced in some weld seam designs during expansion. A plastic joint generally exhibits a greater strength in shear than in peel. It would be desirable to take advantage of the greater shear strength in order to reduce the likelihood of a joint separation.
Therefore, there is a need for a bolster design and manufacturing process that provides the advantages of injection molding while reducing inefficiency and joint separation. Likewise, such a design and manufacturing process would be beneficial for other robust hollow articles, such as pressure vessels.